1. Field of the Invention
The present invention relates to photovoltaic devices, and, more particularly, to solar cells including photoelectric conversion layers composed of polycrystalline silicon (polycrystalline silicon solar cells). The present invention also relates to a structure of a photovoltaic device in which, while preventing a decrease in the open-circuit voltage, in particular, due to the use of a texture structure in the photovoltaic device, the short-circuit current can be improved and the conversion efficiency can be improved.
2. Description of the Related Art
Since a presentation made on solar cells including photoelectric conversion layers composed of microcrystalline silicon (microcrystalline silicon solar cells) at the University of Neuchatel, Switzerland, in 1996, microcrystalline silicon solar cells have been receiving attention (refer to Material Research Society Symposium Proceedings vol. 420, 1996 “ON THE WAY TOWARDS HIGH EFFICIENCY THIN FILM SILICON SOLAR CELLS BY THE MICROMORPH CONCEPT” J. Meier, A. Shah et al. (Non-patent Document 1)). The reason for this is that use of microcrystalline silicon for photoelectric conversion layers increases the short-circuit current, and by stacking the photoelectric conversion layers together with amorphous silicon layers, a high conversion efficiency can be achieved. Furthermore, in microcrystalline silicon solar cells, the light-induced degradation phenomenon, which is a known problem in amorphous silicon, is not observed.
Herein, the term “microcrystalline silicon” means small-grain-size silicon. There are two views on the relationship between microcrystalline silicon and polycrystalline silicon. In one view, microcrystalline silicon and polycrystalline silicon are clearly differentiated. In another view, microcrystalline silicon is considered to be a type of polycrystalline silicon. The former is based on the understanding that microcrystalline silicon is a material that is not considered as an extension of the theory of polycrystalline silicon. The latter is based on the understanding that microcrystalline silicon is small-grain-size polycrystalline silicon.
In this specification and the claims, the latter view is adopted. Therefore, “polycrystalline silicon” is a concept that naturally involves “microcrystalline silicon”. In this specification and the claims, “polycrystal” means an assemblage of many crystal grains. In this specification and the claims, polycrystals are defined to have an average crystal grain size of 5 nm to 100 μm. Among these, those which have an average crystal grain size of 5 nm to 5 μm are defined as microcrystals. The average crystal grain size can be determined using the Scherrer equation from the full-width at half-maximum of the (220) peak obtained by X-ray diffraction analysis.
Furthermore, Sharp Technical Journal No. 83, August 2002 “Crystalline silicon thin-film solar cells” (Non-patent Document 2) describes observations on the relationship between the irregularities of the first electrode and the open-circuit voltage (Voc), and the relationship between the irregular shape and the mechanism of grain boundary generation.
On a planar substrate, since polycrystalline silicon grows substantially perpendicular to the surface of the substrate, grain boundaries occur substantially perpendicular to the surface of the substrate. In such a case, photocarriers moving substantially perpendicular to the surface of the substrate do not substantially pass transversely across the grain boundaries. On the other hand, on an irregular substrate, since polycrystalline silicon grows substantially perpendicular to the inclined planes of the irregularities, crystal grains generated from adjacent inclined planes collide with each other, and many grain boundaries occur in random directions. In such a case, since photocarriers must pass through many such grain boundaries, the Voc and the fill factor (F.F.) decrease.
As described above, when a first electrode having a texture structure suitable for polycrystalline silicon solar cells is used, it is necessary to take crystal growth into consideration.
Japanese Patent Laid-Open No. 2002-151715 (Patent Document 1) discloses a technique in which by specifying the shape of irregularities of a first electrode, the (220) orientation of polycrystalline silicon is controlled. According to this method, tin oxide and zinc oxide are used for the first electrode, and the following measurement results are obtained: Voc 0.525 V, Jsc 22.8 mA/cm2, and conversion efficiency 8.44%. However, in this method, the short-circuit current is not very large, and the light collection efficiency is not sufficient.
Furthermore, Japanese Patent Laid-Open No. 2002-299660 (Patent Document 2) discloses a technique in which the conversion efficiency is improved by specifying the angles of projections on the surface of a substrate. In this method, since the angles of polygonal-pyramid-shaped projections on the surface of a first electrode (which is described as a back electrode in the specification) range from 8° to 40°, it is possible to suppress a leakage current due to the generation of grain boundaries. However, it is assumed that the open-circuit voltage is not as high as that in the case of a planar substrate. Furthermore, since the first electrode has a surface with the projections described above and the thickness of a crystalline silicon layer (which is described as a semiconductor layer in the specification) is large, a second electrode layer (which is described as a transparent conductive layer in the specification) is planar, and thus light collection efficiency is not sufficient.
Japanese Patent No. 2771667 (Patent Document 3) discloses a technique in which projections are formed on a substrate by a laser method or a photolithographic method, amorphous silicon is deposited only on the projections, and then, using the amorphous silicon as a nucleus, a polycrystalline silicon layer is grown by thermal CVD. However, the laser method or the photolithographic method is an expensive process, which leads to an increase in the cost of the photovoltaic device.